Sedimentation is a physical process frequently used to separate settleable suspended material from a liquid as for example in the treatment of sewage, industrial wastes, process water or drinking water. A tube settler is a device used to accomplish sedimentation consisting of a multiplicity of long and narrow tubes in a systematic alignment. A tube settler operates under two fundamental principles. The first is that laminar flow can be easily maintained in the long passages of small cross-sectional area that characterize a tube settler. Sedimentation rates in a tube settler under conditions of laminar flow are essentially unchanged from sedimentation rates under quiescent conditions. In many conventional sedimentation basins and clarifiers however, sedimentation may be hindered by inlet and outlet turbulence, thermal currents, wind induced currents and inherent unequal flow distribution patterns within the tanks. The effects of these hinderances are a reduction of particles' settling velocities and a reduced true hydraulic retention time.
The second principle of tube settler operation is that a reduction of settling distance will reduce sedimentation time. For a particle settling at its terminal fall velocity, sedimentation time is directly proportional to the vertical settling distance. Since the vertical settling distance in tube settlers is usually about 2 to 4 inches as compared to approximately 12 feet in conventional sedimentation basins and clarifiers, sedimentation time is substantially reduced.
Tube settlers which employ continuous settled solids removal are sharply inclined to provide a downward solids flow which is generally countercurrent to the upward liquid flow. In this countercurrent regime, treated liquid exits at the top end of the settler while the settled solids are discharged at the bottom end.
Many different cross-sectional shapes have been used in tube settler design. The relative efficiencies of various designs have been reported by Anderson et al., U.S. Pat. No. 3,768,648 and Tanabe et al., U.S. Pat. No. 4,122,017. Efficiency of tube settlers is evaluated on the basis of suspended solids removal over a wide range of flow rates. Generally effluent quality will remain at an acceptable level as flow through the tubes is increased until a critical flow rate is reached. As the flow is increased beyond the critical flow rate, effluent quality deteriorates at an accelerated pace. The magnitude of the critical flow rate is dependent on the nature of the suspended material as well as the configuration of the tubular passages. A liquid with suspended solids that settle rapidly will exhibit a higher critical flow rate than the same liquid with suspended solids which settle slowly.
The critical flow rate is probably produced by a combination of many physical factors including: (1) insufficient time for complete settling of the settleable material, (2) an increase in the resuspension of already settled material because of the destablization of countercurrent solids flow, and (3) hindered settling due to localized turbulence at the interface of the countercurrent flow. A high critical flow rate correlates with greater efficiency in that a tube settler can handle a higher throughput rate without a significant loss of effluent quality. And a tube settler with a high critical flow rate will demonstrate better effluent quality at flows beyond the critical flow rate.
Anderson et al., showed that a settling tube with a chevron configuration in an orientation with the central apex directed downward was more efficient than circular, hexagonal, diamond and square shaped tubes. Improved efficiency was attributed to a uniform maximum settling distance within the chevron tube and a concentration of solids along the central collection groove at the bottom of the chevron tube.
Tanabe et al., demonstrated a modification of the chevron tube design utilized by Anderson et al. which was termed a boomerang configuration. In this design which approximates the shape of a boomerang, the vertical side walls found in the chevron of Anderson et al. are more outwardly disposed toward their lower ends. This modification according to Tanabe et al. eliminates the acute angle normally found in the chevron design between the tube's top walls and vertical side walls. Increasing this angle reduces localized boundary effects thereby allowing a slightly higher rate of flow in this region of the tube. According to Tanabe et al. the boomerang cross-section permits a higher critical flow rate through the tube resulting in improved treatment efficiency, as demonstrated by increased turbidity removal at flow rates exceeding the critical flow rate of the chevron.
An analysis of liquid velocity gradients present in tubes with either a chevron or boomerang cross-section shows that the highest flow rate occurs between the center apices of the top and bottom walls directly above and in close proximity to the central groove where settled solids are collected. A large velocity gradient is created between the point of maximum liquid velocity and the countercurrent flow of solids, adversely affecting the efficiency of both of these tubes. The large sheer stress exerted by the upflowing liquid on the settled solids opposes the gravitational force acting to pull the settled solids downward. Consequently the settled solids are hindered in their downward movement and may be subject to resuspension.